This invention is directed to a method and apparatus for molding a plastic bottle based on the hot-blow or one-step technique for manufacture of molecularly-oriented plastic bottles. More particularly, it is directed to improvements in a hot-blow or one-step machine system for the manufacture of plastic bottles comprising a given number of parison-forming stations, i.e., sixteen (16); a given but lesser number of blowing stations, i.e., six (6); and a storage area between the parison-forming and parison-blowing stations, permitting a high degree of parison mold and blow mold utilization and overall efficiency; and to the improved method of manufacture.
In recent years substantial effort has been directed to the formation of molecularly-oriented plastic bottles as a replacement, or partial replacement, for glass bottles. According to the prior art, the manufacture of molecularly-oriented bottles have utilized either a reheat, the so-called two-stage, process and system or a hot-blow, the so-called one-stage, process and system. In the reheat or two-stage process, parisons are first injection molded in a parison mold, cooled to room temperature, and stored for subsequent blowing into the finished bottle in a blow-mold. At time of blowing, the parisons are reheated to the blowing temperature of the plastic prior to blowing. In the hot-blow or one-stage process and system, the parison is injection molded in a parison mold and, substantially immediately after formation, is transferred while at the blowing temperature to a blow mold where the parison is blown into the finished bottle.
Each of the above-noted systems and processes have advantages and disadvantages. In the reheat process and system, efficient and effective use can be made of both the parison forming and the blowing stations which need not be integrated. However, substantial thermal energy is lost during the total operation in that the parison, after formation, is cooled down during storage and then reheated at the time of blowing. Moreover, the parisons from storage must be fed to the blow station, duplicating handling. The hot-blow process and system eliminates heat loss and duplicate handling. However, the advantages of the hot-blow process and system are mitigated in that conventionally in order to have proper temperature conditioning of the parison at the blowing station, for each parison station there is a corresponding blowing station. Since the time required for parison formation, including temperature conditioning, is substantially longer than for parison blowing, i.e., approximately twenty seconds for parison formation versus five seconds for parison blowing for a polyethylene terephthalate (PET) bottle, the blowing stations are only inefficiently used.
As described in the aforesaid U.S. Pat. No. 4,140,464 and copending application Ser. No. 013,417, it is now recognized that there can be a substantial delay between parison formation and the blowing of the parison without detriment to the properties of the finished bottle. For example, tests have established that there is no significant change in tensile yield values for delays in blowing of the parison after parison formation of up to ninety seconds and longer; and, furthermore, that temperature distribution in the walls of the parison varies only slightly over substantial periods. Based in part on the aforesaid recognition, a process and machine system is described in the aforementioned U.S. Pat. No. 4,140,464 and in copending application Ser. No. 013,417 whereby the number of blow molds is lower than the number of parison or injection molds. A parison storage area is provided between the parison forming and blowing stations.
The present invention is directed to a modification in the earlier machine systems and in the process of operation. In accordance with the alternate design, the parisons are withdrawn from the storage area sequentially and continuously, and heat-conditioned sequentially and continuously, if required, and blown sequentially and continuously in blow molds fewer in number than the number of parison-forming stations. The machine system of the alternate design includes means for transferring the parisons from the parison storage area to a continuously moving conveyor system transporting the parisons through a heat-conditioning device having means to rotate the parisons during the heat-conditioning period, conveying the parisons to a rotary infeed mechanism and delivering the parison into a continuous rotary multi-station blow molding mechanism in which split blow molds actuate to encompass the received parison, engaging a stretch rod and blow head to stretch and blow the bottle. Upon completion of the blowing cycle, the bottle is removed from the blow mold by rotary take-out means, delivering the completed bottle to a discharge station. The modifications in accordance with this invention will be more fully apparent from the following drawing and detailed description of the invention and, as will be apparent, are primarily in the mechanisms for transferring the hot parison from the storage unit to the blow stations.